In recent years, optical elements that have a refractive index decreasing or increasing continuously in a direction from the optical axis toward the periphery and thereby function as a convex lens or a concave lens have been proposed (see, for example, Patent Document 1 and Patent Document 2).
For example, Patent Document 2 discloses a lens having a radial refractive index distribution by replacing Na ions in a glass with Ag ions in a molten salt. However, because of limited processability, the minimum diameter of the lens is about 1 mm, and the maximum difference in the refractive index in the radial direction is about 0.1. The lens must therefore have a large thickness (for example, about 3 to 10 mm) to achieve a sufficient converging or diverging function. This prevents miniaturization or integration of the optical element.
A material having a refractive index varying in the direction of the optical axis rather than in the radial direction has been proposed. An optical element formed of the material and having a thickness decreasing or increasing continuously in a direction from the optical axis toward the periphery can reduce the converging or diverging aberration (see, for example, Patent Document 3 and Non-patent Document 1).
For example, in Non-patent Document 1, glass layers having different refractive indices are stacked and are heated to form a lens having a refractive index distribution of about 0.1 in the thickness direction. However, because of limited processability, the lens must have a large thickness (for example, about 1 to 10 mm). This prevents miniaturization or integration of the optical element.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 54-109456.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-159702.
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2001-281417.
Non-patent Document 1: TOYAMA Syoji, “KOJIKU HOUKOU BUNPU KUSSETURITU KOGAKU ZAIRYO GRADIUMU (An optical material GRADIUMU having a refractive index distribution in the direction of the optical axial),” O plus E, New Technology Communications Co., Ltd., March 1998, pp. 330-336.